This invention relates to a battery pack that can prevent battery short circuits when disconnected from electrical equipment.
Battery packs have an electrode structure that is much more susceptible to shorting than single battery cells. This is because the + and - electrodes are disposed close to each other. Further, since a battery pack connects a plurality of battery cells in series to increase the output voltage, the short circuit current is large. If battery pack electrode terminals are shorted, and excessive current flows, battery performance is markedly degraded. Still further, Joule heating of the batteries and the short circuiting metal gives rise to dangerous conditions.
Battery packs that will not short circuit when disconnected from electrical equipment have been developed to avoid these dangers. These battery packs are disclosed in the following Japanese Patent Publications:
1 Japanese utility Model Publication No. 59-19336 issued Jun. 4, 1984; PA1 2 Japanese Non-examined Utility Model Publication No. 4-14861 issued Feb. 5, 1992; PA1 3 Japanese Non-examined Utility Model Publication No. 4-47257 issued Apr. 22, 1992; and PA1 4 Japanese Non-examined Utility Model Publication No. 63-87769 issued Jun. 8, 1988. PA1 1 a cover over the electrode terminals; PA1 2 an internal reed switch; and PA1 3, 4 an internal switch with mechanically movable contacts.
Disclosure 1 cites a battery pack provided with shutters to cover the battery terminals when disconnected from electrical equipment. When the battery pack is attached to electrical equipment, the shutters are moved out of the way from the surfaces of the electrode terminals. Hence, the electrode terminals can connect with the power supply terminals of the electrical equipment. When the battery pack is detached from the electrical equipment, the electrode terminals are covered by shutters and short circuits due to metal contacting the terminals is prevented.
Disclosure 2 cites a battery pack which has an internal reed switch. The electrical equipment has an internal magnet to control the reed switch. The reed switch turns on in close proximity to the electrical equipment magnet and turns off when separated from the magnet. When a battery pack with this structure is attached to electrical equipment, the reed switch is turned on by the magnet and electric power is supplied from the batteries to the electrical equipment. Since the reed switch is turned off when the battery pack is detached from the electrical equipment, the batteries cannot short circuit even if metal contacts the electrode terminals.
Disclosure 3 cites a battery pack which has a leaf switch connected in series with the batteries. The leaf switch has a lever which projects out from the casing of the battery pack. When the lever is pushed the leaf switch is turned on, and when the lever is not pushed the leaf switch is off. When the battery pack is attached to electrical equipment, the lever is pushed, the leaf switch is turned on, and electric power is supplied from the batteries to the electrical equipment. When the battery pack is removed from the electrical equipment, the lever is no longer pushed, and the leaf switch is turned off. For this reason, when the battery pack is disconnected from the electrical equipment, the batteries will not short circuit even if metal contacts the electrode terminals.
Finally, disclosure 4 cites a battery pack which has a connection switch with mechanically movable contacts in series with the batteries. The connection switch has two metal plates which can elastically deform. The metal plates are disposed in close proximity and are arranged to be pushed upon by the opening in the casing. When the metal plates are pushed upon by the opening, they make contact to turn the connection switch on. When the metal plates are not pushed upon, they elastically deform and separate to turn the connection switch off. Electrical equipment that use this type of battery pack have projections from the casing opening to apply pressure to the metal plates. Consequently, when the battery pack is attached to the electrical equipment, pressure is applied to the metal plates and the connection switch is turned on, but when the battery pack is removed from the electrical equipment, no pressure is applied to the metal plates and the connection switch is turned off. For this reason, when the battery pack is not attached to the electrical equipment, the batteries cannot short circuit even if metal contacts the electrode terminals.
The prior art battery packs cited in the above disclosures protect against short circuits with three types of structures:
Since type 1 battery packs are provided with shutters that can slide along the casing, the structure to cover the electrode terminals is complex and expensive. Further, if the shutters malfunction and fail to slide properly, the electrode terminals will not be reliably covered when disconnected from the electrical equipment, and the danger of electrical shorting will exist. On the other hand, failure of the shutters to move to their proper position can result in inability to correctly attach the battery pack to the electrical equipment. Consequently, this configuration has the drawbacks that casing structure is complicated and reliable movement of the shutters over long periods is difficult to obtain.
Since battery packs with configuration 2 use reed switches, the above drawbacks are eliminated. In particular, since the reed switch contacts are enclosed in a hermetic glass case, these battery packs have the feature that switching can be performed in an ideal environment with the ambient atmosphere shut out from the contacts. This is because the contacts can be activated by magnetic force. However, since the reed switch contacts are switched by a magnet, it is difficult to obtain a structure capable of switching large currents. This is because it is difficult to make large contacts push together strongly and also reliably separate. Therefore, even though the reed switch can be used effectively for battery packs with low load currents, reed switch lifetime presents a problem when used in battery packs with large charging and discharging currents. In particular, there is no way to use the reed switch in a battery pack containing high capacity batteries which are rapidly charged with high currents.
High current battery packs can use type 3, 4 structures because strong pressure is applied to mechanically movable contacts. However, when strong pressure is applied to the switches movable contacts, the opposing reaction force acts on the battery pack-electrical equipment attachment region. This is because one part of the electrical equipment pushes strongly against the battery pack movable contacts when the battery pack is attached. Consequently, it is necessary to provide a sturdy battery pack-electrical equipment attachment region for this type of battery pack. Since the battery pack attaches to the electrical equipment with one part of the electrical equipment pushing strongly against the battery pack movable contacts, considerable force is required to attach the battery pack. It is also difficult to provide an easily detachable structure since the battery pack and electrical equipment are firmly attached. Further, a structure that switches by pressure on movable contacts has the drawback that contact pressure degrades with use. This is because the shape of the movable contacts gradually deforms with repeated long term application of strong pressure. Therefore, reliable long term operation of the movable contact switch is difficult to obtain, and durability is a problem.